It has been conventional practice to provide gas turbine aeroengines with simple bobweight governors of a hydromechanical type in order to prevent hazardous overspeeds of one or more of the spools of the aeroengine, the arrangement being such that when the rotational speed of the governed spool exceeds a certain predetermined limit, the governor mechanism effects reduction of the fuel supply to the engine and thereby reduces the speed of the spool to an acceptable value. With the recent introduction of digital electronic fuel controllers for gas turbine aeroengines, the latest design practice is tending to implement the function of overspeed limiting electronically by means of digital computers. However, there remains a case for implementing the overspeed limiting function by the traditional hydromechanical governor means for at least the high pressure (and therefore high speed) spool of a multispool gas turbine aeroengine. This is because a simple hydromechanical governor can act as a high-reliability back-up speed limiting device for the whole engine; even if electronic speed limiter(s) fail to control the speed of the lower pressure spool(s) due, e.g. to damage sustained by the electronic components, the fact that the high pressure spool is also speed limited will under most circumstances limit the speed of the other spools(s) too.
A problem inherent in speed governing devices of the above type, which exert control over the engine only when other controls have failed, is that although they are highly reliable, it is nevertheless possible for them to experience a so-called "dormant failure", that is, some sort of breakage or other fault which occurs at a time when the device in question is not participating in control of the engine, the failure only being apparent when the device fails to operate satisfactorily when required. To overcome this problem it is known to schedule regular manual checks or resets of the device and to carry out ground runs of the engine for observation of the device's correct working. Clearly, operators of the engine would prefer not to have to do this and elimination of such activities would result in increased economy of operation.
It is an object of the present invention to provide the operators of gas turbine aeroengines, particular those having digital electronic fuel control systems, with an automatic means of assuring themselves that a mechanical speed governing device installed on the engine has not suffered from a dormant failure.
Accordingly, the present invention provides a self-checking governor arrangement for controlling the rotational speed of a spool in a gas turbine engine, comprising:
a displaceable member and means for effecting its displacement by an amount which varies with said spool speed;
valve means for controlling fuel flow to the engine, the valve means being actuatable by the displaceable member to reduce fuel flow to the engine in dependance upon said displacement of said displaceable member;
switch means actuatable by the displaceable member to output a switch signal whenever said displaceable member is displaced by a predetermined amount corresponding to a predetermined spool speed within a predetermined normal range of spool speeds experienced by said spool during normal operation of said engine;
means for generating a spool speed signal whose value varies with said rotational speed of said spool; and
data processing means connected to receive said switch signal and said spool speed signal, said data processing means being adapted to determine whether receipt of the switch signal substantially coincides with receipt of a value of the spool speed signal corresponding to said predetermined spool speed and to output a governor status signal dependent upon the result of said determination.
In this way, the correct operation of the governor mechanism can be automatically checked during run-up of the engine before every takeoff, assuming of course that the above-mentioned predetermined spool speed is suitably chosen.
The above-mentioned means for effecting displacement of the displaceable member by an amount which varies with the engine spool speed conveniently comprises bias means acting against said displacement and a rotary mechanism driven from said spool for displacing the displaceable member against said bias means by an amount directly related to the spool rotational speed.
The invention is particularly useful as an overspeed governor and in this case the valve means is arranged such that it is actuated by the displaceable member to reduce fuel flow only when displacement of the displaceable member attains a large predetermined amount indicative of a spool speed beyond said predetermined normal range of spool speeds.
The switch means comprises a switching device of some sort, such as a microswitch, and any associated circuitry necessary to produce a signal suitable for use as the switch signal for input to the data processing means.
The data processing means may be adapted to output a signal indicative of satisfactory operation of the governor arrangement whenever said receipt of said switch signal occurs substantially simultaneously with said receipt of said value of said spool speed signal. Alternatively, the data processing means may be adapted to output a signal indicative of unsatisfactory operation of the governor arrangement whenever receipt of said switch signal does not occur substantially simultaneously with said receipt of said value of said spool speed signal.
It is convenient if the self-checking governor arrangement is part of a main fuel control system of the engine, the data processing for checking of the governor function being performed in a digital electronic controller forming part of the main fuel control system.
It is preferred that the displaceable member be part of a bobweight governor mechanism.
Most current gas turbine aeroengines are of the two or three spool type and usually the invention will be applied to a governor controlling the high pressure spool of the engine.